Keeping a South African
Sasolburg, a town in the Free State Province, South Africa, is a vital link inthe country’s economy. Having been established since the 1950s, the town haslong been of importance to the country as its main center for the petrochemicalindustry, hosting major refining facilities such as the NATREF (NationalPetroleum Refiners of South Africa (Pty.) Ltd. Oil Refinery).
During 2001, management at the NATREF facility identified that the refinery’sBlowdown Sewer, in the main pipe rack in the main process area, had severestructural defects over a length of approximately 190 m. The defects includedcorrosion of the pipe invert, collapsed sections and debris that was obstructingthe flow through the pipe. The pipe itself was known to be located at depthsbetween 4.5 and 6 m below ground level and to run in among numerous other buriedsewers, pipes, cables and other structures.
NATREF has different categories of runoff water and effluent streams that areseparated into distinct underground sewer systems, allowing for specifictreatment methods per stream. The Blowdown Sewer is one of the four vitalgravity sewer systems removing water and processed effluent from the mainprocess area and carries return water from the steam and waste heat boilers,which is used to clean out chemical build-ups in the tubes and drums in therefining process. The chemical constituency of the wastewater passing throughthis sewer comprises a mixture of phosphates, silica and iron. The liquid ishighly alkaline, running at a pH of 11.5. Normally the temperature of the liquidis between 70 and 120 C and flows peak at up to 20 l/s. Any replacement pipewould have to cope with highly aggressive liquid and high thermal shocksituations continuously.
After an initial evaluation, undertaken by the project’s engineeringconsultant, Goba Moahloli Keeve Steyn (Pty.) Ltd. (GMKS) with project engineerBrian de Swardt, it was evident that the rehabilitation of the pipe would needcollective agreement from all NATREF departments regarding the pipe materials,installation methods and safety. To this end, various technical meetings wereheld to establish appropriate options and to determine a workable and practicalsolution for the rehabilitation.
It was established at an early stage that the NATREF requirements could noteasily be supported by technology then currently available in South Africa.Various options were evaluated in terms of new technology that had recently beenbrought into the country and other feasible options availableinternationally.
In November 2003, NATREF finally agreed that the rehabilitation shouldinvolve the construction of a parallel pipe using trenchless technologytechniques. The approved pipe should be a chemically-resistant clay jacking pipewith Viton seals and stainless steel couplings. The pipe would be installedutilizing directional, multi-stage horizontal auger boring (also known aspilot/auger microtunneling), which was suitable for the soil conditions known toexist at the site and which provided the desired installation accuracy. Alllaunch and receiving pits would be strategically placed to serve as both manholeaccesses and suitable environmental monitoring points subsequent to thecompletion of the pipeline installation.
Geology
During the design phase,geotechnical information was obtained from the results of previous pilingexercises that had been completed at NATREF in the vicinity of the new works.Additional soil penetration tests were completed to verify the consistency ofthe backfill material encountered. Some limited open-excavation work was done toexpose backfill material during a visit by Dr. Uffmann from Bohrtec to verifythe suitability of the soil conditions for use of the microtunneling machinethat was potentially to be used.
Ground penetrating radar (GPR) inspections were also completed from groundlevel and from within a large diameter storm water pipe, which runs parallel tothe proposed new pipe route. However, the results were inconclusive due to alarge amount of interference within the ground area surveyed.
Ultimately, the ground investigation works showed the virgin ground materialcould be best described as a very moist, brown to orange brown, thinlyhorizontally bedded, slightly micaceous, decomposed arenaceous (sandy) siltstoneor silty fine grained sandstone. The general nature of the material increases toa stiff consistency at depths of 6 m. Excavation of shallow sumps in thelaunch/receiving pits also exposed the existence of gray or gray brown, verystiff material tending to very soft rock, comprising very thinly beddedcarbonaceous shale/siltstone.
The backfill material was a different matter, comprising loose to very loose,non-cohesive material. In certain areas, the backfill also contained railwayballast.
It was decided that the contract for the pipeline installation, known as theRehabilitation of Blowdown Sewer between MH 18 and MH V, was to be completed intwo parts. Part I involved exploratory work to verify the position ofinfrastructure and fully establish the soil conditions. Part II, theconstruction phase, was awarded, after the initial exploratory work revealedthat a new pipe alignment would be possible and that soil conditions weresuitable for the specified technique. On this project, as well as planning,preliminary and detailed design, GMKS undertook the monitoring of theconstruction works. The construction contract for the work was awarded toInsitu-Pipelines cc (South Africa), with Pius Walsh in charge.
As previously mentioned, installation of the new pipeline was to take placewithin the confines of the refinery’s original main pipe rack. The originalconstruction of the main pipe rack had comprised a ‘mass’ open-trenchexcavation, with numerous parallel pipes and chambers being installed atdifferent elevations. Therefore, the new parallel pipe would need to be boredthrough both the existing backfill of the main pipe rack and virginmaterial.
Pipe Design
A vital key to the successof the NATREF Blowdown Sewer replacement project was going to be the pipematerial installed. While investigating the pipe available at that time, it wasfound that none had the capacity to cover both sets ofrequirements.
Following extensive Internet research and recalling contactmade with Granville Bull during a Denlok presentation in Johannesburg a fewyears earlier, GMKS approached Naylor Drainage Ltd. in the United Kingdom, acompany known worldwide as a clay pipe manufacturer, with a view toinvestigating the suitability of the company’s Denlok vitrified clay jackingpipes as a solution to the refinery’s pipeline rehabilitation problem.
However, the Denlok system is not designed for high temperature operations.But some four years earlier, Naylor had acquired the expertise of WestdeutscheSteinzeug Werke GmbH, a German ceramic company. This company had developedexpertise in manufacturing spigot and socket sewer pipes with high chemicalresistance and the capability to withstand high operating temperatures. Nayloragreed to develop a jacking pipe based on the same material and techniques forits installation in situations where such operating extremes occur. Simon Marsh,one of Naylor’s most experienced pipe jacking experts, headed the developmentteam.
After some 18 months of work, including design, testing and production, thenew jacking pipe, to be known as DenChem, was created. From concept to fruition,with major input from de Swardt and a team of specialist material manufacturersand suppliers that worked on the seals and collars and a total commitment to thedevelopment process by Naylor, a pipe suitable for the NATREF project wasavailable. The new pipe was fully certified to both BS EN295-7, the Europeanstandard for vitrified clay pipes and fittings for drains and sewers; and BS EN65, extra chemical resistance of vitrified clay pipes and fittings.
Ultimately, the pipe produced by Naylor for the NATREF contract comprised:DN200 DenChem HT clay jacking pipe with 304 stainless steel sleeves and aparticle board thrust ring. The inside diameter of each pipe was 200 mm and theoutside diameter was 270 mm. The pipe was supplied in 996-mm lengths with Vitonand stainless steel couplings.
The boring machine ultimately chosen for the work was a Bohrtec BM400pilot/auger system. The unit is capable of producing a jacking force of 100tonnes and a maximum torque of 12,000 Nm. The maximum speed of rotationavailable to the cutter head is 120 rpm for core drilling, but this varies,depending on the action being undertaken at the time. Drive lengths up to 80 mcan be completed using this system. The unit operates from either a circular orrectangular start shaft, with minimum dimensions of 2,000 mm.
Given that the DenChem pipe was specifically designed with the NATREF projectin mind, the pipe used for the project was 200 mm in diameter, but the unit canwork with standard pipe diameters up to a maximum of 620-mm OD.
The installation technique used at NATREF was standard in terms of the threephases of piloting, boring and jacking using an auger head with a standard openface, as there were no adverse groundwater problems and no lubrication wasused.
The Bohrtec system works on a basic three-phase installation routine.The first is the pilot bore phase, which establishes an online and on levelsmall bore between the launch and reception shafts. This bore is then upsizedusing the augering phase with the auger head enlarging the pilot bore to therequired diameter as it is advanced by the installation of the auger casingchain. It is through the auger chain that the spoil produced by the reamingaction is then removed. Finally, the pipes themselves are jacked into the nowcorrect diameter bore, expelling the auger casings in to the reception pit. Asthe last pipe is placed, the pipe line is completed.
Installations
In all, the first phase ofthe construction work installed: six lengths, totaling 158 m using themicrotunneling system; a 7-m length of pipe that was to be installed through asteel sleeve that had previously been installed using the pipe rammingtechnique; and some 20 m of open-cut works, including lengths through manholechambers. The length of the individual drives varied from 5 to 48 m. The DenChempipe was used throughout the installation operation, including the pipe rammedand open-cut sections.
As part of the investigatory phase, two inspection pits were excavated forthe validation of soil conditions and position of underground services. Thesewere later used as launch/receiving pits during the microtunneling works. Thestart date for the construction project was Sept. 13, 2004, with a contractedscheduled completion date of June 22, 2005.
As with all complex installation projects of this nature, things did not goas smoothly as might have been hoped and planned.
In the first instance, technically, implementation of the design was hamperedas the pipe alignment had to be altered due to the anticipated position of someservices differing from those indicated on the available plans. There were alsoadditional submerged chambers in the most congested portion of the pipe rackthat were uncovered during the course of planning work that were not shown onthe available as-built plans. As it turned out, these were constructed to dealwith an alignment clash between the refinery’s Oily Storm Water (OSW) and CleanStorm Water (CSW) sewers.
Further to this, during the course of the construction work, progress washampered by the invert of the existing Blowdown Sewer pipe being corrodedentirely, resulting in a high rate of exfiltration from the pipe that causedflooding of the construction excavations. To overcome this, additionalover-pumping facilities had to be installed.
Similarly, the OSW sewer wasfound to have open joints and may also have been structurally damaged in certainareas. Again, this resulted in a high rate of exfiltration from the pipe thatadded to the flooding of the worksite excavations. Again, additionalover-pumping installations were required.
The final problem during the construction phase was that as excavations weremade to accommodate the installation equipment, large cavities were found insome places in the backfill material adjacent to the excavation shoring. Thisresulted in the shoring mechanism becoming compromised. Additional grouting ofvoids and improvements in the shoring design were required to ensure a safeworking environment.
However, despite the additional work and equipment required during theplanning and construction, all upgrade work on the new Blowdown Sewer wassuccessfully completed by Insitu-Pipelines during normal refinery productionperiods.
“The depth and congestion of services, both parallel and crossing theproposed pipe route made it extremely difficult to position suitable launch andreceiving pits,” said de Swardt. “However, once constructed, the alignmentaccuracy achieved by the boring machine proved to be well within tolerance andallowed for the accurate positioning of the new pipe through the congestedservices.
“The key to the success of this project is that the pipe and equipmentsuppliers remained totally committed from the start of the inquiry phase, whichenabled the project engineer to pull together key suppliers and contractors fromdifferent hemispheres of the globe and provide a composite solution to theclient’s problem,” he added.
In terms of the pipe itself, Marsh said: “We have developed specialistexpertise in both microtunneling and aggressive operating environments and ourearly involvement in this scheme meant that we could make a major contributionto the success of this challenging project.”
With the successful completion of this pioneering project, NATREF decided toundertake further work using the same installation technique and pipe material.Known as the Rehabilitation of Underground Sewers — Phase II, the current workcomprises the installation of some 115 m of new pipeline in similar groundconditions upstream of the previous works.
Once again the engineer for the project is de Swardt, but this time as adirector of Industrial and Urban Infrastructure (Pty) Ltd. (South Africa) (IUI),which has been retained to oversee the planning, design and constructionmanagement of Phase II. Furthermore, Insitu-Pipelines cc (South Africa) hasagain been retained as the contractor for the work. The current project began inlate November 2005, with a scheduled completion date of June 2, 2006.
Ian Clarke is a freelance writer and owner of No-Dig Media, based in theUnited Kingdom.
